1. Field of Invention
The invention concerns a droplet mist generator and, in particular, a droplet mist generator as apart of a burner.
2. Summary of the Invention
Micro-droplet mist generators for producing individual droplets on call are known in ink printing. In EP-O 713 773 a droplet mist generator with piezoelectric flexural transducers and a nozzle each under the transducer is proposed in which the individual transducers with partition walls are separated from each other so that when the transducer is deflected from the true path, a droplet is ejected from the nozzle assigned to another transducer.
From the older German patent application with the file number 19507978.7 a dosing system for fuel dosing is known that has numerous micro-nozzles and electrothermic, electrostatic, electrodynamic, or piezoelectric transducers with which an expansion of vapor bubbles in a fuel-filled chamber or a change in volume of this chamber is effected by means of an electrical trigger signal, therefore making it suitable for the repeated ejection of fuel droplets that are essentially of the same size. The use of a piezoelectric membrane actuator is described as a preferred transducer principle.
When using the expansion of vapor bubbles as an actuator principle for dosing traditional types of fuel, the various components of the fuel vaporize under very different conditions. The vaporization therefore does not occur abruptly enough to achieve an efficient formation of droplets. Variations in the composition of the fuel lead, in addition, to irregularities so that reliable dosing or transport is not possible when using the vapor bubbles principle. Transducers in which the chamber volume is changed are complicated structures. In the case of a piezoelectric flexural transducer, for example, a piezoelectric ceramic element is covered with a membrane that forms a chamber wall. This is necessary to obtain the change in volume, because when a piezoelectric crystal expands in a direction, there is always a vertical contraction connected with it. In the piezoelectric flexural transducer and the membrane, material must be deformed during a large-scale deflection from the true path so that works of deformation must be carried out against strong inner mechanical resistance. Such transducers therefore work with a poor degree of effectiveness. And in relation to the structural size of the transducer elements, only a small dispersion is attained due to the resistance. A high acceleration of fluid also cannot be obtained.
By using the invention, the problem of creating an inexpensive pump with a small structural size in which a stream of fluid in the form of a cloud of droplets can be dosed with a high flow rate while maintaining a certain droplet size and density is solved.
The problem is solved according to the invention by a droplet mist generator. The droplet mist generator comprises a pump chamber, which is constructed in a casing and is connected to a fluid reservoir; a nozzle area constructed in the casing wall, having a plurality of nozzles; a plate-shaped piezoelectric flexural transducer that is positioned in the pump chamber and attached so that it overhangs and can be bent around a quadrature axis running transversely to the direction of the overhang; openings that are constructed between the edges of the piezoelectric flexural transducer, which form ends in the direction of its quadrature axis, and the casing wall; and a control system through which voltage impulses can be applied to the piezoelectric flexural transducer by bending the piezoelectric flexural transducer, driving out fluids, and ejecting droplets from the nozzles of the nozzle area.
With the idea of impacting an entire area of nozzles with a piezoelectric flexural transducer positioned so it is effectively fluidic inside a chamber filled with fluid, a droplet mist generator with an especially high flow rate is created, whereby the droplet size and density can be determined with the form of the nozzle area and by means of the length, strength, and frequency of the pulse emitted by the control system.
Piezoelectric flexural transducers produce an especially high deflection from the true path when accelerating quickly and can be operated with high frequencies. In addition, they have only a small inner mechanical resistance. Using the piezoelectric flexural transducer principle, a high conversion rate of electrical to mechanical energy can be obtained with respect to the structural size. Moreover, piezoelectric flexural transducers are simple constructions and thus are inexpensive and reliable.
The special arrangement of the transducer and the numerous nozzles leads to the fact that the transformed mechanical energy can be used for the production and transport of the droplet stream with a high degree of efficiency. By transforming the energy directly near the nozzles on which the droplets are formed, a high share of fluidic energy is supplied for the formation of droplets and their transport.
The fluidic losses due to the compression of the fluid are, moreover, minimized because the transformer surface, in front of which a peak pressure is produced during the impacting action, with the nozzle areas faces a large nozzle cross-sectional area, through which a conversion of the produced pressure takes place during transport by forming and ejecting droplets. In other words, a large share of the generated pressure is transformed.
Through the high acceleration of the piezoelectric flexural transducer the entire energy is supplied to the droplets forming on the nozzle in the shortest time span, which leads to an abrupt breaking off of the droplets while preventing a larger back-flow into the chamber.
The opening between the edges of the piezoelectric flexural transducer and the casing wall allows the fluid to stream around the piezoelectric flexural transducer during the backward movement of the piezoelectric flexural transducer so that the increasing volumes between the piezoelectric flexural transducer and the nozzle area are filled with the fluid that is flowing back and no air is pulled into the nozzles in the chamber. The openings are therefore calculated to be so large that fluidic resistance that occurs due to friction remains small enough that the deflection from the true path is not greatly impaired. At the same time, the openings are calculated so they are so small that during the rapid impacting action of the piezoelectric flexural transducer the fluid located in front of the transducer cannot be carried off quickly enough through the opening and is pushed through the nozzles.
The voltage pulses given off by the control system are coordinated in such a way that the transport of fluid is made possible. The impacting action, which causes the ejection of droplets through the nozzle, can occur considerably more quickly than the backward movement of the piezoelectric flexural transducer so that during the impacting action no streaming occurs through the opening in which the backward flow runs against a sufficiently strong stream. For the purposes of the present invention, a known control system can be used.
By using a single piezoelectric flexural transducer to impact several nozzles, the system is inexpensive and not very prone to problems.
According to the invention the chamber and fluid reserve can be connected to any suitable place in the chamber. Preferred, however, is a connecting line on one of the sides of the piezoelectric flexural transducer turned away from the nozzle area. If one does not completely reduce the volume of the chamber, but reduces the volume between the piezoelectric flexural transducer and the nozzles, when the volume on the opposite side is raised, fluid can be drawn from the fluid reserve connected to the pump chamber while the droplets are ejected. In so doing one can obtain especially short repeat times between the successive surges or bending and droplet-ejection operations, as a result of which the transport performance is raised even more.
According to the invention the chamber can be connected to the fluid reserve by means of a line or other connection. Preferably, however, the chamber is connected to the fluid reserve through several lines, especially two lines. In so doing, the droplet mist generator can be degassed during operation by providing fluid through a connecting line and carrying away gas and fluid through the other connection lines. Moreover, an improved and quicker fluid feed can be obtained with a majority of lines, each in a suitable arrangement, which leads to a shortened refill time between two droplet-producing pulses.
According to the invention the connections between the chamber and fluid reserve can be designed so there is as little resistance as possible. Preferred are, however, choke sites in the connections that provide that the least possible fluid is driven through the feed lines that connect the chamber with the fluid reserve, thus guaranteeing that the transport performance of the droplet mist generator is high. Preferably the choke sites are designed in such a way that the fluid goes against a high fluidic resistance during a high pressure impulse when a droplet is ejected, while with a small difference in pressure during the refill operation the fluid goes against only a small fluidic resistance and thus the spray frequency can be increased. Flap valves can also be provided in the connections so that a streaming of fluid into the chamber through the connection is made possible while at the same time preventing the fluid from streaming out.
According to the invention the nozzles can be designed as cylinder-shaped channels, openings, channels with square cross-sectional areas, or channels of any other shape; and they can have a constant channel cross section. They can also be designed so they taper toward the chamber. It is, however, preferable that they are designed so they taper in the direction away from the chamber. In so doing, the cross-sectional area of the nozzle with the smallest diameter is obtained on the opening of the nozzles in the surrounding environment. Because bordering surfaces between two fluids constantly strive to take on the state with the least energy in the smallest area of the boundary surface, a nozzle tapering outward leads to a situation in which the edge of the meniscus between the fluid and gaseous environment constantly strives to remain on the outer edge of the nozzle. By reducing the extent of the change in the position of the meniscus edge, the droplet mist generator is guaranteed to work in an especially robust way, which leads to a higher transport performance because no outfall cycles result.
According to the invention, the outer side of the casing wall in the part of the casing wall in which the nozzle field is positioned can be made of any suitable material. Preferred, however, is a coating with teflon or with another suitable anti-adhesive material. With such a coating one prevents the outer side from being moistened, i.e., a moving forward of the 3-phase boundary between fluid, gaseous surroundings and the casing structure results from opening the nozzle. As a consequence, the meniscus edge remains at the end of the nozzle toward the outside during the formation of the droplets, as a result of which the invention is guaranteed to work in a robust fashion with a high transport performance.
According to the invention the droplet mist generator can have any suitable piezoelectric flexural transducer. Preferably, however, the piezoelectric flexural transducer is a multiple-layer piezoelectric ceramic transducer with an additional passive piezoelectric layer. In so doing, the same deflection of the piezoelectric flexural transducer can be obtained with a small control voltage. This has the advantage that the regulations for the maximum voltage can be observed with many possible uses of the droplet mist generator without limiting the productivity.
According to the invention the droplet mist generator can have only one piezoelectric flexural transducer and only one nozzle area. According to the invention a majority of piezoelectric flexural converters and/or a majority of nozzle areas can be provided in the droplet mist generator. In this connection several piezoelectric flexural transducers are arranged in such a way that their plate surfaces can be positioned in a plane next to one another or their plate surfaces can be positioned in various levels so they overlap each other or are positioned next to each other. In a preferred form of the model an arrangement is provided with a second piezoelectric flexural transducer and a second nozzle area that lie across from the free end of the first piezoelectric flexural transducer and that are essentially mirror-inverted to the first piezoelectric flexural transducer and the first nozzle area. The control system in this case is constructed in such a way that the piezoelectric flexural transducer and the second piezoelectric flexural transducer can be controlled by various pulse frequencies, pulse length, and/or pulse phases. The arrangement of the two piezoelectric flexural transducers lying across from one another with the same control of the piezoelectric flexural transducers leads to a situation in which the fluid, which is driven out to the other piezoelectric flexural transducer, is subject to fluidic resistance due to the incoming fluid forced out of the other piezoelectric flexural transducer. As a result, a higher pressure can build up and the transport flow rate can be increased. By using a control with shifted pulse phase the transport flow rate can be varied. A control can also be carried out with various pulse frequencies and/or pulse lengths. A variation or different control with respect to one or more of the parameters pulse frequency, pulse length, and pulse phase can also be used with a set nozzle arrangement in the nozzle area to vary the droplet size and droplet speed.
According to the invention the nozzle area can be designed in any suitable part of the casing wall. In an especially preferred form the nozzle area is designed in a part of the casing wall that is positioned inside the overhang of the plate surface of the piezoelectric flexural transducer in the direction in which the free end of the piezoelectric flexural transducer is movable when passing through its equilibrium position. The nozzles of the nozzle area are thus essentially positioned in such a way that all the nozzles would be covered by the transducer surface if one would move the piezoelectric flexural transducer up to the part of the casing wall in which the nozzles are constructed. In this working model an opening of a suitable size is designed between the free end of the piezoelectric flexural transducer and the part of the casing wall lying across from it in the extension of the transducer.
According to the invention any suitable distance or no distance at all may separate the piezoelectric flexural transducer from the part of the casing wall in which the nozzle area is designed. In a preferred form of the model when the piezoelectric flexural transducer is in its equilibrium position, a small distance between the piezoelectric flexural transducer and the part of the casing wall in which the nozzle area is designed is formed. In this case the piezoelectric flexural transducer can be moved away from the nozzle area by applying a voltage pulse and then moved back to the nozzle area by applying a reverse polarized voltage or using mechanical restoring forces, whereby the droplet ejection is effected. If the distance is chosen to be small enough, overshooting the equilibrium position when moving it back can lead to a situation in which the piezoelectric flexural transducer hits against the casing wall in which the nozzle area is constructed. The piezoelectric flexural element can, however, be moved away by applying the voltage pulse immediately in the direction toward the nozzle area so that the droplet ejection can be started directly when applying the voltage pulse. In this case as well the piezoelectric element hits against the casing wall. This bumping against the casing wall can have the advantageous effect that the acceleration of fluid is quite abruptly broken off, resulting in an especially regular and quick break off of the droplets. How strong this effect is can depend upon how the piezoelectric flexural transducer and the part of the casing wall in which the nozzle area is constructed are formed. If there are plane surfaces, contact will occur to a great extent across the entire surface; if there are arched surfaces or non-plane surfaces shaped in another form, contact occurs only at one or a few places.
The opening between the free end of the piezoelectric flexural transformer and the casing wall lying opposite it in the extension of the piezoelectric flexural transformer can have any width according to the invention. Preferably, however, it is not more than five times as large as the gap that occurs when the piezoelectric flexural transformer is in equilibrium position when no voltage is applied.
In another preferred form of the model the piezoelectric flexural transformer in its equilibrium position, which occurs when no voltage is applied, lies on the part of the casing wall in which the nozzle area is constructed and the piezoelectric flexural transformer is moved away from the nozzle field by applying voltage by using the control system. In this case the formation of droplets is triggered when the piezoelectric flexural transducer springs back after the voltage pulse ends by applying a reverse voltage impulse or mechanical restoring force.
According to the invention the part of the casing wall in which the nozzle area is constructed can be constructed like the other parts of the casing wall. Preferably the part of the casing wall nonetheless projects into the chamber. Such a form has the advantage that high pressure, which builds up in the gap that becomes more and more narrow as the surface of the piezoelectric flexural transducer is moved to the casing wall, builds up only in the area in which it falls when the droplets emerge from the nozzles and thus can be utilized. As a result, there is a reduction of the fluidic losses during the droplet ejection operation and thus an increase of the transport performance and the efficiency of the pump. An advantageous effect is also obtained when the fluid is refilled from the reservoir. The narrow distance between the piezoelectric flexural transformer and the casing wall, in which fluid can only flow against a high fluidic resistance, is shorter compared to a form of the model without casing wall parts designed to project into the chamber. As a consequence, the necessary fluid can be drawn back more quickly and the droplet production frequency and the transport quantity can be further increased.
In another preferred form of the model the nozzle area is positioned so it lies across from the free end of the piezoelectric flexural transformer in the extension of the piezoelectric flexural transformer. In this way the nozzle area is staggered a little bit with respect to the free end of the piezoelectric flexural transformer. The nozzles are thus, preferably, in the direction of overhang of the piezoelectric flexural transformer. Such an arrangement has the advantage that it is possible, given an especially small construction size, to arrange a majority of the piezoelectric flexural transformers in the direction of the plate surface one after the other or inside the plate surface plane next to each other, whereby each piezoelectric flexural transformer can be assigned to a corresponding nozzle area without having to further enlarge the construction area required to set up the piezoelectric flexural transformer due to the nozzle area. Preferably, in this arrangement when the piezoelectric flexural transformer is in its equilibrium position, there is a gap between the piezoelectric flexural transformer and the next wall lying vertical to the plate surface of the piezoelectric flexural transformer.
According to the invention the droplet mist generator is a droplet mist generator for any suitable fluids. In this connection the droplet mist generator according to the invention can be used separately or as a component of any suitable system. Preferably, the droplet mist generator is nonetheless a component of a burner, whereby the fluid reserve is a fluid fuel reserve. The nozzles of the nozzle area serving as burner nozzles have a smallest diameter of at least 10 .mu.m and at most 100 .mu.m. As a result, droplet sizes are obtained that are especially well-suited for the production of an inflammable mixture made of fuel droplets and a gaseous oxidant. With traditional fluid fuels such as diesel fuel or gasoline, such droplet sizes lead to a situation in which the fuel droplets completely evaporate right after the ejection from the nozzle, resulting in an inflammable and/or highly combustible mixture. Depending on the viscosity and transport quantity, the nozzles according to the invention have diameters larger than 100 .mu.m corresponding to the fluidic requirements.
According to the invention the mid-points of each of the neighboring nozzles of the nozzle area that serve as burner nozzles have any suitable distance between them. Preferably, the mid-points nonetheless occur at intervals of at least 50 .mu.m and at most 2,000 .mu.m. By choosing the distances between the neighboring nozzles in this arrangement one obtains a further improvement of the fuel-oxidant mixture, and with it a further increase in the burner performance.
According to the invention the droplet mist generator can have any number of nozzles depending on its use. Preferably, however, the droplet mist generator has at least 50 nozzles. With at least 50 nozzles or more a burner is especially well suited for use as a burner for vehicle heating or household heating devices.
In another preferred form of the model holes are provided in the piezoelectric flexural transducer according to the invention to reduce the fluidic resistance of the piezoelectric flexural transducer. In yet other forms of the model valves according to the invention can be provided in the droplet mist generator with which the transport of fluid is possible even with larger nozzle diameters. In this connection the invention provides that either droplets or a continuous stream of fluid is transported. Preferably, the operation of existing valves is carried out with a piezoelectric flexural transducer that simultaneously converts the fluidic energy. According to the invention the chamber on the nozzles can also be sealed off against its surroundings by bringing the piezoelectric flexural transducer into a certain position.